Imipenem monohydrate is the N-formimidoyl derivative of thienamycin, and has the structural Formula I
It is the first clinically available member of new class of β-lactam antibiotics that possess the carbapenem ring system. Imipenem exhibits an extremely broad spectrum of activity against gram-positive and gram-negative aerobic and anaerobic species, which is partly due to its high stability in the presence of β-lactamases.
Imipenem was first disclosed in U.S. Pat. No. 4,194,047 and was obtained by lyophilization technique. The product obtained by lyophilization is found to be largely amorphous and stated to be thermodynamically unstable. The process also involves an initial purification through column chromatography using hydrophobic resins.
A thermodynamically stable crystalline monohydrate form of imipenem is disclosed in U.S. Pat. No. 4,260,543 which is obtained by crystallization of a lyophilized sample of imipenem. However, this process is not satisfactory on a commercial scale as it requires isolation of the product by column chromatography, lyophilization, followed by crystallization. Moreover, the prolonged process for isolation of the final product leads to degradation of imipenem, thus affecting the purity of the product.
U.S. Pat. No. 4,292,436 discloses crystalline imipenem by purifying the crude product by column chromatography. Another method for preparing imipenem having a high degree of crystallinity by freeze crystallization process has been reported by Connolly et. al. in J. Pharm. Sci, 85, 174(1996). However, these processes are tedious, cumbersome and unsuitable for industrial use.
Our pending PCT application No. PCT/IB02/01718 provides a process for the isolation of crystalline imipenem monohydrate from a solution containing imipenem without using lyophilization, freeze drying or chromatographic techniques at any stage.
However, it has been observed that imipenem obtained by methods which do not involve column chromatography contains polymeric and colored impurities. The polymeric impurities are side products formed during the synthesis of imipenem, which crystallize out along with imipenem. These impurities do not have any ultra-violet absorption and hence, are difficult to detect. Although, the polymeric impurities are not reflected in the qualitative determination of purity by HPLC, but the quantitative determination (Assay) shows that the product has about 5-10% of these impurities.
The colored impurities are degradation products of imipenem formed during production or under storage, imipenem being inherently unstable in solution as well as sensitive to heat and light. These colored impurities adversely affect the appearance of imipenem, which may appear from pale yellow to brownish powder instead of the desired white crystalline powder.
The purification of imipenem is difficult due to its unstable nature. The crystalline imipenem has relatively low solubility in water at room temperature. A purification process thus requires dissolving imipenem in large volumes of water. The chromatographic purification also requires eluting with large volumes of water. The process of recovering the purified product is uneconomical as it requires concentrating water at low temperature thus necessitating the use of lyophilization, reverse osmosis, or freeze drying techniques.